In the most general case an actuator of a helical rotary pump represents pair-wise interacting helical rotors disposed into an encircling chamber. The tooth profile of rotor helical thread can have different shapes: an ellipse and an envelope in the inventor's certificate SU125860, an involute and a special conjugate curve in the inventor's certificate SU 1032255, a set of involutes producing a quasi-cycloidal profile in the inventor's certificate SU 292044. The tooth contact of rotors is accompanied by great slippage in actuators with the said tooth profiles, causing great friction losses and reducing their durability. A helical pump is known with an actuator representing a cage with chambers where two helical rotors are mounted (SU 1751408). Each rotor has one helical tooth of a cycloidal shape. Working areas of the tooth addendum at cross-section are produced (here) along an epicycloid and conjugated areas of the tooth dedendum are drawn by a hypocycloid. Helical teeth of rotors are conjugated with each other and one of rotors is driving and the other is driven. Rotation from the driving rotor to the driven one is transmitted by synchronizing pinions mounted on rotor shafts, thus increasing overall dimensions of the pump and complicating its layout.
The patent RU 2062907 describes a pump with an actuator also having two single-thread rotors with synchronizing pinions. In order to create smoother force profile, segments of epy- and hypocycloids are conjugated by means of an involute.
A two-screw pump is known for conveying high-viscosity media according to the patent RU 92489. Helical rotors in its pressure chamber are made double-thread, that is, with two cycloidal teeth. Such a shape of teeth gives the tight contact between rotors at any angle of rotation, thus providing leak resistance. As in the previous pump, torque transmission from one rotor to another is provided by means of synchronizing pinions. Because of the tight contact considerable friction forces appear between rotors, decreasing the pump efficiency and increasing its wear and reducing its lifetime.
Screw pumps with cycloidal rotors are known (see Zhmud A. E. Screw pumps with cycloidal engagement.—M.: Mashiz, 1963). Theory and technology of manufacturing stated in the book are applicable to screw pumps designing with any number of rotors. Cycloidal pumps with three double-thread rotors are the most common A driving helical rotor has two teeth with convex cycloidal profile at its cross-section. Two driven rotors arranged at both sides from the driving one have two concave cycloidal teeth with sharp edges. Geometrical relations of helical threads are chosen to provide leak resistance of actuators when the torque is not transmitted, that is, there is a slotted clearance between teeth. When rotors rotate the slotted clearance will be displaced along the tooth height and rotor flanks will possess different speed within the slot area, since rotors with cycloidal teeth are always slipping with respect to each other. This difference in flow speeds at rotor flanks causes cavitation limiting the rotor rotational speed. Synchronization of rotors is provided only due to medium pressure and this medium is inhomogeneous (for example, gas inclusions in liquid medium), this synchronization will be broken thus leading to the leakage, the power contact of rotors and the wear increase. Such contact is especially harmful for driven rotors with sharp edges. In order to prevent rapid wear, sharp edges of driven rotors are abated by one or two chamfers (see also RU 2215189). Moreover, the slotted clearance can provide leak resistance only for liquids with definite flow characteristics. When pumping high-flow liquids the pump will have great reverse leakages decreasing its productivity abruptly. This pump is unsuitable for operation in media with little solid inclusions, since because of slippage they are entrapped by the slot and when displacing across the tooth they create transverse valleys on rotor flanks. That is why such a pump can be applied for pumping rather viscous, thick and homogeneous media without solid inclusions.
Therefore, independently on a shape of helical teeth, rotors for all the said pumps are produced to exclude the power contact between rotors and rotation of driven rotors is provided either by additional synchronizing pinions or due to the pumped liquid pressure. This is explained by the fact that the power contact of rotors, firstly, limits the lifetime of an actuator because of increased friction forces in engagement, secondly, limits the rotor rotation speed due to torque pulsation. With increase of the rotor rotation speed, overall dimensions and weight of a machine are decreased at other equal conditions.
The said book (see p. 26) states that the design of a helical pump with cycloidal engagement possesses reversibility, that is, it can operate as an engine, including a hydraulic rotating servomotor. Therefore, we can speak about this mechanism as a helical rotary machine with one and the same actuator as helical rotors engaging pair-wise and being disposed into an encircling chamber. The tooth profile of one of the rotors in the pair is generated in its cross-section by convex segments of an epicycloid and the tooth profile of the other rotor in the said pair is generated by concave segments of an epicycloid with the slotted contact between them. The slotted contact provides mutual leak resistance of screws seal for homogeneous liquids with definite flow characteristics. The said actuator with cycloidal rotors and the slotted contact between rotors is chosen as a prototype. The drawback of the prototype, as it was stated above, is limitation of rotor rotation speed and limitation of working media characteristics.
Therefore, the task of creating a helical rotary machine with high productivity at long lifetime and high efficiency is still urgent.